When the brakes are applied on a vehicle traveling at a given velocity, braking torques are generated at each of the braked wheels. The braking torque causes a retarding or braking force to be generated at the interface between the tire and the driving surface. The braking forces generated at the wheels then cause a decrease in the vehicle velocity.
Ideally, the braking forces at the wheels increase proportionately as the driver increases the force on the brake pedal. Unfortunately, this is not always the case in braking procedures. As the braking torque and hence the braking force at the wheel is increased, the rotational speed of the braked wheels becomes less than the speed of the vehicle. When the rotational speed of a wheel is less than the vehicle speed, "slippage" is said to occur between the tire and the driving surface. This slippage, when severe, can lead to lock-up of a wheel and skidding of the vehicle. In most cases, lock-up causes an increased required stopping distance. Lock-up also causes a degradation in directional control due to a reduction in the lateral forces at the wheels.
Both of these problems associated with lock-up were addressed with the advent of anti-lock brake systems (ABS). A basic anti-lock brake system uses sensors to monitor the velocity at one or more of the wheels, decides whether the wheel is at or approaching an excessive wheel slip condition based on these velocity measurements, and modulates the braking pressure accordingly to avoid lock-up. The ABS aids in retaining vehicle stability and steerability while providing shorter stopping distances.
One method by which an excessive wheel slip condition is identified in the ABS is comparing the velocity of each wheel to a reference speed. The reference speed is an estimate of the true vehicle speed based on current and previous values of the individual wheel velocities. If the velocity of a wheel is significantly less than the reference speed, then the wheel is deemed by the ABS to be excessively slipping. The ABS then reduces the pressure actuating the brake in order to reduce brake torque. The reduction of brake torque causes a reduction of the braking force, which then causes a reduction of the slip in the wheel.
After a period of constant braking pressure following the pressure reduction, the pressure actuating the brake is increased until excessive wheel slip occurs again. The cycle of decreasing the brake pressure, maintaining constant brake pressure, and then increasing brake pressure is repeated until excessive slip no longer occurs. The parameters which define the specifies of this cycle depend on both the vehicle and the driving surface conditions.
A false indication of excessive slip can be generated by the ABS by a superfluous reference speed, in other words, an overestimated reference speed. In order to illustrate how an overestimated reference speed is formed, a sequence known as "spin-up and brake" is considered for a four wheel drive vehicle. FIG. 1 shows an example time sequence of throttle and brake commands which would cause the spin-up and brake. With the vehicle initially traveling at a given velocity, an increased throttle command 10 is given (indicated by the dashed line), thus causing the driven wheels to accelerate. If an excessively high acceleration, based on the load on the tire and the coefficient of friction at the interface between the tire and the driving surface, is commanded, the tires on the driven wheels begin to slip on the driving surface. This results in the speed of the vehicle not increasing as rapidly as the increase in the rotational speed of the driven wheels. The resulting duration of slippage between the tire and the driving surface is known as the spin-up phase 12.
After the spin-up phase 12, the throttle is assumed to be released, and then a brake command 14 is given (indicated by the solid line). The time period corresponding to the application of the brake command 14 is known as the brake phase 16.
FIG. 2 shows a wheel speed 20, reference speed 22, and actual vehicle speed 24 resulting from the throttle and brake commands given in FIG. 1. The wheel speed 20 (indicated by the solid line) is increasing at a higher rate than the actual vehicle speed 24 (indicated by the dotted line) during the spin-up phase 12. The reference speed 22 (indicated by the dashed line), used to estimate the actual vehicle speed 24 based on the wheel speed 20, typically has its rate of increase constrained by a ramp growth limiting mechanism. The ramp growth limiting mechanism constrains the rate of increase of the reference speed 22 to be less than or equal to the rate of increase of the wheel speed 20 during the spin-up phase 12. However, the reference speed 22 increases at a higher rate than the actual vehicle speed 24 during spin-up.
After the throttle is released, the wheel speed 20 decreases down to approximately the same value as the actual vehicle speed 24. However, the reference speed 22 remains near its value at the end of the spin-up phase 12. Thus, during the braking phase 16, the ABS detects a significant difference 26 between the reference speed 22 and the wheel speed 20. Since the detection of excessive slip in the ABS is based on the difference between the reference speed 22 and the wheel speed 20, the ABS falsely determines that excessive slip is present. The ABS would then begin to modulate brake pressures to prevent excessive slip, even though excessive slip is not present.
Overestimation of reference speed can also occur in other circumstances. Rapid vehicle decceleration without wheel slippage can lead to an overestimated speed reference. Such deceleration could occur during vehicle impact with a stationary or slowly moving object.
Previous prior art methods have focused on the prevention of undue growth of vehicle speed reference signals during spin-up. The major shortcomings of these methods are their difficulty in tracking slow spin-ups and correcting the reference speed resulting from an extended spin-up.